This invention relates to the treatment of the surface of aluminum vacuum brazing sheet for improving its fillet-forming capability during vacuum brazing. The surface treatment involves light etching of the sheet surface with a mixture of aqueous HF and HNO.sub.3.
Aluminum is widely employed in the automotive industry as a material of construction and one area of use involves its application in the manufacture of automotive heat exchangers, such as radiators. In the manufacture of radiators it is necessary to join different parts together and most frequently such joining is accomplished through brazing. Brazing generally provides good bonding between the parts to be joined provided the surfaces to be joined are clean. In the case of aluminum parts to be joined by brazing it is necessary to disrupt or remove the tenacious oxide surface layer from the aluminum parts in order to achieve good bonding. In the past oxide surface layers were generally removed during flux or non-vacuum brazing by the application of different fluxes together with the brazing alloy. The most commonly used fluxes for such purpose were composed of mixtures of metal chlorides and fluorides, such as mixtures of NaCl-KCl-LiCl with ZnCl.sub.2, KF-AlF.sub.3 or other inorganic fluorides. When the chloride-containing flux mixtures are applied to the aluminum surfaces to be brazed it becomes necessary to remove residual, post-braze flux by thorough washing of the treated surfaces. This is to assure the removal of all residual flux from the surface which otherwise could cause corrosion of the brazed assembly.
To eliminate the need of washing off the flux remaining on the treated aluminum surface, the use of potassium tetrafluoroaluminate (KAlF.sub.4), a complex fluoridic salt, was recommended. This salt can be obtained by the mixing of equimolar quantities of potassium fluoride (KF) and aluminum fluoride (AlF.sub.3). This complex salt is often applied as slurry to the surfaces of the parts to be joined by brazing then it is dried and heated until it melts. This molten salt is capable of removing any aluminum oxide film coating the surfaces to be joined by brazing; but any salt remaining on the surface after brazing will no attack the aluminum metal due to its insolubility in water. Although this type of fluxing material has many advantages over the chloride salt mixtures, it is difficult to apply evenly to the surfaces to be joined, particularly, when the surfaces to be joined belong to complex heat exchangers.
The following U.S. patents represent references which relate to the use of various solid fluoride-containing fluxes which generate water-insoluble residues on the surface of the brazed joints: U.S. Pat. Nos. 3,951,328, 4,224,086, 4,556,165 and 4,579,605.
To avoid the difficulties associated with the use of the water-insoluble fluxes U.S. Pat. No. 4,619,716 recommends the use of an aqueous solution containing KHF.sub.2. This solution, when in contact with aluminum, provides a chemical conversion coating of K.sub.2 AlF.sub.5 on the surfaces to be joined by brazing. The aqueous solution used for the preparation of the KHF.sub.2 can also be made from a mixture of KF and HF. Although the surface preparation method disclosed in this U.S. patent provides an improved coating method in comparison to the use of KALF.sub.4 alone, the method described still requires careful preparation of the flux and a residual flux coating still remains on the surface of the brazed joints.
It has also been recommended in U.S. Pat. No. 3,779,839 to etch the surfaces to be vacuum brazed with a solution containing: (a) a fluoride compound; (b) a chlorine-containing salt; and (c) CrO.sub.3.sup.- ion, in order to improve the fillet-forming ability. Although there is scant solid residue left behind after the etching treatment, the presence of CrO.sub.3 ions and the chloridic compound may create corrosion problems subsequent to brazing unless the surfaces are thoroughly washed after etching. The use and disposal of this etchant may also create environmental problems due to the CrO.sub.3 ion content.
It has now been discovered that etching of the aluminum surfaces to be joined by vacuum brazing with a highly dilute aqueous solution of HF and HNO.sub.3 eliminates the need for a water-soluble or insoluble flux or etching with a fluoride-chloride solution in the presence of CrO.sub.3. The produced brazed joints not only exhibit improved strength due to the increased fillet-forming ability but also improved appearance due to the absence of solid flux or harsh etching agents.